NASA Tests Atomic Clock for Deep Space Navigation

A glimpse of the Deep Space Atomic Clock in the middle bay of the General Atomics Orbital Test Bed spacecraft. Image Credit: NASA› Larger view

In deep space, accurate timekeeping is vital to navigation, but
many spacecraft lack precise timepieces on board. For 20 years, NASA's Jet
Propulsion Laboratory in Pasadena, California, has been perfecting a clock.
It's not a wristwatch; not something you could buy at a store. It's the Deep Space Atomic Clock (DSAC), an
instrument perfect for deep space exploration.

Currently, most missions rely on ground-based antennas paired
with atomic clocks for navigation. Ground antennas send narrowly focused
signals to spacecraft, which, in turn, return the signal. NASA uses the
difference in time between sending a signal and receiving a response to
calculate the spacecraft's location, velocity and path.

This method, though reliable, could be made much more efficient.
For example, a ground station must wait for the spacecraft to return a signal,
so a station can only track one spacecraft at a time. This requires spacecraft
to wait for navigation commands from Earth rather than making those decisions
on board and in real-time.

"Navigating in deep space requires measuring vast distances
using our knowledge of how radio signals propagate in space," said Todd
Ely of JPL, DSAC's principal investigator. "Navigating routinely requires
distance measurements accurate to a meter or better. Since radio signals travel
at the speed of light, that means we need to measure their time-of-flight to a
precision of a few nanoseconds. Atomic clocks have done this routinely on the
ground for decades. Doing this in space is what DSAC is all about."

The DSAC project aims to provide accurate onboard timekeeping for
future NASA missions. Spacecraft using this new technology would no longer have
to rely on two-way tracking. A spacecraft could use a signal sent from Earth to
calculate position without returning the signal and waiting for commands from
the ground, a process that can take hours. Timely location data and onboard
control allow for more efficient operations, more precise maneuvering and
adjustments to unexpected situations.

This paradigm shift enables spacecraft to focus on mission
objectives rather than adjusting their position to point antennas earthward to
close a link for two-way tracking.

Additionally, this innovation would allow ground stations to
track multiple satellites at once near crowded areas like Mars. In certain
scenarios, the accuracy of that tracking data would exceed traditional methods
by a factor of five.

DSAC is an advanced prototype of a small, low-mass atomic clock
based on mercury-ion trap technology. The atomic clocks at ground stations in
NASA's Deep Space Network are about the size
of a small refrigerator. DSAC is about the size of a four-slice toaster, and
could be further miniaturized for future missions.

The DSAC test flight will take this technology from the
laboratory to the space environment. While in orbit, the DSAC mission will use
the navigation signals from U.S. GPS coupled with precise knowledge
of GPS satellite orbits and clocks to confirm DSAC's performance. The
demonstration should confirm that DSAC can maintain time accuracy to better
than two nanoseconds (.000000002 seconds) over a day, with a goal of achieving
0.3 nanosecond accuracy.

Once DSAC has proved its mettle, future missions can use its
technology enhancements. The clock promises increased tracking data quantity
and improved tracking data quality. Coupling DSAC with onboard radio navigation
could ensure that future exploration missions have the navigation data needed
to traverse the solar system.

Technologies aboard DSAC could also improve GPS clock stability
and, in turn, the service GPS provides to users worldwide. Ground-based test
results have shown DSAC to be upwards of 50 times more stable than the atomic
clocks currently flown on GPS. DSAC promises to be the most stable navigation
space clock ever flown.

"We have lofty goals for improving deep space navigation and
science using DSAC," said Ely. "It could have a real and immediate
impact for everyone here on Earth if it's used to ensure the availability and
continued performance of the GPS system."

DSAC is a partnership between NASA's Space Technology Mission
Directorate and the Space Communications and Navigation program office, a
program under the Human Exploration and Operations Mission Directorate. DSAC
will launch in 2018 as a hosted payload on General Atomic's Orbital Test Bed
spacecraft aboard the U.S. Air Force Space Technology Program (STP-2) mission.